1. Field of the Invention
The present invention relates to an imaging device provided with an image-forming optical system and an image sensor, and a mobile information terminal incorporating the imaging device.
2. Description Related to the Prior Art
A digital camera has been incorporated into various mobile devices such as a mobile phone and a PDA. Furthermore, the digital camera has been widely used for a videophone and a vehicle camera or an onboard camera allowing a driver to monitor an image of the inside or the outside of a vehicle. The digital camera has an imaging device and a peripheral circuit device. The imaging device is a unit of an image-forming optical system and a solid state image sensor. The solid state image sensor photoelectrically converts a subject image formed by the image-forming optical system. The peripheral circuit device drives the solid state image sensor and reads an imaging signal therefrom and performs various signal processes to the imaging signal to obtain a digital image signal. The peripheral circuit device then stores the digital image signal.
For example, to incorporate the digital camera into the mobile phone or the PDA commonly used as the mobile information terminal, it is relatively easy to reduce a width of the peripheral circuit device using various mounting techniques. On the other hand, it is difficult to reduce a width of the imaging device without affecting its optical performance. It is possible to reduce the width of the imaging device simply by reducing a total length of the image-forming optical system. However, when the image-forming optical system has a short width, an incidence angle of a principal ray on each point, on an image forming plane, away from an optical axis becomes larger than that in an image-forming optical system with a long length. This is due to a short geometric distance between an aperture stop plane and an image forming plane, and is unavoidable due to optical design limitations.
On the other hand, a front side illumination CMOS sensor commonly used as the solid state image sensor has a microlens arranged on its incident surface. Thereby, aperture efficiency of each pixel is improved to direct as much image-forming light as possible to its photoelectric conversion portion. Thus, the aperture efficiency and photoelectric conversion efficiency of the front side illumination CMOS sensor are improved compared to an image sensor with no microlens. The front side illumination CMOS sensor photoelectrically converts the normal incident light efficiently. However, because the photoelectric conversion portion is located under a wiring layer, obliquely incident light is reflected by the wiring layer before it reaches the photoelectric conversion portion or leaks to adjacent pixels. Thus, the obliquely incident light cannot contribute to the photoelectric conversion of the pixel. Each pixel has the highest photoelectric conversion efficiency when light is incident vertically on the imaging surface. The photoelectric conversion efficiency of light incident at an angle of the order of 20° is drastically reduced to the order of 35%, taking the conversion efficiency of the normal incident light as 100%. When the incidence angle exceeds the order of 30°, most of the light is reflected or blocked by the wiring layer and the like or leaks to adjacent pixels. Thereby, the photoelectric conversion efficiency is reduced to 5% of that of the normal incidence.
As described above, because the photoelectric conversion portion is located under the wiring layer in each pixel of the front side illumination CMOS sensor, a tolerance range of a light diffusion angle is small. When an angle of incidence (incidence angle) is larger than the vertical incidence angle (=0°), light leaks to the adjacent pixels immediately. Thus, the sensitivity is reduced. Light with a large incidence angle increases as being closer to the periphery of the image. As a result, the light available for the photoelectric conversion is reduced in the periphery of the image. Thus, the peripheral area of an image becomes relatively dark compared to a screen center thereof. In other words, the CMOS sensor cannot reproduce an image based on the brightness indicated by the aperture efficiency and supposed to be ensured by the optical system. Generally, the image with dark peripheral portion is corrected by image processing afterwards. However, the correction causes deterioration of SN ratio. When an extremely dark image is corrected excessively, the deterioration of the SN ratio exceeds a tolerable range and thus rendering the image unusable. In the color CMOS sensor, the light leaking to the adjacent pixels causes color mixture. The color mixture can also be corrected in the image processing. This correction also causes the deterioration of the SN ratio. When an image with extreme color mixture is corrected excessively, the deterioration of the SN ratio exceeds a tolerable range and thus rendering the image unusable. For these reasons, the front side illumination CMOS sensor is not suitable for a small imaging device.
To correct the incidence angle of the principal ray in the periphery of the image, a microlens pitch may be set smaller than a pixel pitch (that is, the so-called scaling). Ideally, the angle of principal ray is corrected to zero which is equivalent to vertical incident angle. Thereby, darkening of the illumination in the periphery of the image is reduced. However, in reality, the scaling is not perfect and sufficient to solve the problem. The microlens has its aberration which causes light loss. Also, microlens scaling is not perfectly matched with optics because chief ray angle variation is not linear along with image height. This imperfection becomes significant as principal ray angle becomes larger. For this reason, an optical system for use in the current imaging device is optically designed such that the incident angle of principal ray on the imaging surface is limited to approximately 30° or less as disclosed in U.S. Patent Application Publication No. 2007/0070525 (corresponding to Japanese Patent Laid-Open Publication No. 2007-122007), U.S. Patent Application Publication No. 2008/0266676 (corresponding to Japanese Patent Laid-Open Publication No. 2008-268946), and U.S. Patent Application Publication No. 2008/0180814 (corresponding to Japanese Patent Laid-Open Publication No. 2008-185687).
Due to structural limitation to the solid state image sensor, the conventional imaging optical system requires to set the maximum incidence angle of the principal ray to be the order of 30° on the image forming plane of the optical system. The imaging forming plane of the optical system coincides with the incident surface of the solid state image sensor. The total length of the optical system is 4 mm or more as disclosed in the U.S. Patent Application Publication No. 2007/0070525 and the No. 2008/0266676. Accordingly, further thinning of the imaging device is desired. A total length (with the equivalent air distance or reduced distance) of the optical system disclosed in the U.S. Application Publication No. 2008/0180814 is reduced to the order of 3.2 mm. However, the incidence angle of the principal ray on the image forming plane is required to be less than the order of 30°, which results in insufficient resolution and small image size. Thus, the conventional image sensor has limited application and lacks versatility.